1. Field of the Invention
The present invention relates to a planar type diode, and more specifically to a diode which can be effectively used for electrostatic protection of input/output signals in integrated circuits.
2. Description of Related Art
Conventionally, diodes used for electrostatic protection of input/output signals in integrated circuits have been required to have the following electric characteristics:
(1) In order to protect the integrated circuit from being broken by a static electricity, the breakdown voltage of the electrostatic protection diode is lower than a breakdown voltage of the integrated circuit;
(2) In order to avoid disturbance of the input/output signal, a capacitance of the electrostatic protection diode (between opposite terminals of the electrostatic protection diode) is small, and the breakdown voltage of the electrostatic protection diode is higher than a voltage of a normal input/output signal; and
(3) In order to minimize a consumed electric power, a leakage current of the electrostatic protection diode is smaller than the voltage of a normal input/output signal
One example of the conventional electrostatic protection diode has been constituted of a simple PN junction. In this case, a Zener breakdown is dominant, and therefore, the leakage current of the electrostatic protection diode becomes large if the required breakdown voltage becomes low. On the other hand, an operating resistance is large. For example, when the electrostatic protection diode for a 3V-signal type integrated circuit is realized of the conventional PN junction diode, a diode having its breakdown voltage of 4V has its leakage current on the order of several milliamperes at 3V. Therefore, the electrostatic protection diode for integrated circuits having an input/output signal voltage not smaller than 5V could not have been realized of the conventional diode, because of its leakage current.
An electrostatic protection diode having an NPN structure has been known. A breakdown voltage of this type diode is determined by a punch through voltage, and therefore, this type diode can have an improved leakage current. However, it has been difficult to control the breakdown voltage, and in addition, the breakdown voltage variation in the same wafer is large. Because of this reason, this type diode is not suitable to mass production.